UX: make responsive_post_image_sizes a visible site setting

This is useful for sites that want to cut bandwidth by decreasing
fidelity of thumbnails.

config/locales/server.en.yml

@@ -1163,6 +1163,7 @@ en:
     delete_removed_posts_after: "Posts removed by the author will be automatically deleted after (n) hours. If set to 0, posts will be deleted immediately."
     max_image_width: "Maximum thumbnail width of images in a post"
     max_image_height: "Maximum thumbnail height of images in a post"
+    responsive_post_image_sizes: "Resize lightbox preview images to allow for high DPI screens of the following pixel ratios. Remove all values to disable responsive images."
     fixed_category_positions: "If checked, you will be able to arrange categories into a fixed order. If unchecked, categories are listed in order of activity."
     fixed_category_positions_on_create: "If checked, category ordering will be maintained on topic creation dialog (requires fixed_category_positions)."
     add_rel_nofollow_to_user_content: "Add rel nofollow to all submitted user content, except for internal links (including parent domains). If you change this, you must rebake all posts with: \"rake posts:rebake\""

config/site_settings.yml

@@ -916,7 +916,8 @@ files:
     list_type: compact
   responsive_post_image_sizes:
     default: "1|1.5|2"
-    hidden: true
+    type: list
+    list_type: compact
   crawl_images:
     default: true
   max_image_width: